The invention relates to a sewing machine with a stop motion for bobbin thread.
U.S. Pat. No. 4,188,902 discloses a sewing machine having a thread monitor with a light source and a light receiver arranged on the back of a shuttle. The loop taking body as well as a bobbin case received therein and a bobbin flange turned toward the thread monitor, the flange having an opening for the entrance and exit of control light rays, while the other bobbin flange bears a reflection surface. On the outer side of the loop taking body a second reflection surface, furnishing reference light rays, is applied. The light rays emitted by the light source are distributed by mirrors over the two reflection surfaces or respectively are collected after reflection and supplied to the light receiver shifted in time. The light receiver is connected to a control circuit by which the intensity of the signals formed from the control rays and from the reference rays is compared. As soon as the control rays reach a predeterminable intensity, the control circuit sends a warning signal announcing the end of the thread.
This setup, however, requires the adaptation both of the loop taking body and of the bobbin case as well as of the bobbin to the mode of operation of the thread monitor. In particular, the adaptation of the loop taking body is of great disadvantage if the thread monitor is retrofitted, as the changing of the loop taker body requires adjustment of the sewing machine.
German utility model No. 85 16 211 discloses a loop taker controllable by an opoelectronically thread monitor. For the entrance and exit of light rays separate inlet and outlet openings are provided in the bobbin case and in one of the bobbin flanges. The other bobbing flange is made to be reflecting. The outlet opening of the bobbin is machined in the truncated coneshaped bobbin hub and is relatively small.
Owing to this, the reflected rays can issue only after even the last layer of thread has been partially unwound. The quantity of residual thread is thus determinable with relative precision, but the arrangement operates with one signal only, the absolute magnitude of which must be picked up exactly.